Abstract

Experimental materials

A three-layer plywood sample, produced in Zhejiang, was chosen as our experimental material. The essential parameters of this three-layer plywood are presented in Table 1. The plywood was cut in round pieces (60 mm in diameter) for the rapid detection method and in rectangular pieces (1,000 by 500 mm) for the climate chamber method. After the specimen edges were sealed with aluminum foil to prevent the release of VOCs, they were stored in Teflon bags and placed in the refrigerator until needed.

Table 1 Essential parameters of the three-layer plywood.

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Experimental equipment

The following equipment was used in the experiments. (1) A Micro-Chamber/Thermal Extractor M-CTE250(T)(i) (Markes International Inc., UK) was used with a TY9700 digital hygrothermograph (Tianyue Environmental Protection Technology Co., Ltd., Beijing, China). The carrier gas was nitrogen, with a temperature range of 0°C to 250°C. (2) A 1-m³ VOC/formaldehyde climate chamber (Simplewell Scientific Co., Dongguan, China) was used with a Zhiheng IAQ-Pro Constant Flow Air Sampling Pump (Sensidyne, USA). (3) A TP-5000 Thermal Analysis Desorber (Beifen Instrument Technical Co., Beijing, China) was used. The carrier gas was nitrogen, with the following settings for related parameters: resolution temperature, 45°C; pipeline temperature, 100.0°C; thermal analysis time, 5 minutes; injection time, 1 minute. (4) A DSQ II series GC-MS (Thermo Scientific, Germany) was used. Chromatography was performed with a DB-5 quartz capillary column (3,000 mm by 0.26 mm by 0.25 μm). The chromatographic column was initially at 40°C for 2 minutes, it was then elevated to 50°C (at 2°C/min) and held for 4 minutes, and finally, it was elevated to 250°C (at 10°C/min) and held for 8 minutes. The injection port temperature was 250°C, the distributary flux was 30 ml/min, and the distributary ratio was 30. The following GC-MS conditions were applied: ionization mode, electron ionization (EI); transmission line temperature, 270°C; ion source temperature, 230°C; mass scan range, 50 to 650 atomic mass units (amu). (5) Tenax-TA sampling tubes were obtained from Beifen Tianpu Instrument Technology Co., Ltd. (Beijing, China). (6) Cylindrical stainless steel volumetric flasks were used (100 mm in diameter, 650 mm in height).

Experimental design

Rapid detection method

The experiment used the single-factor method to test the VOC emissions of three-layer plywood under different experimental conditions (the conditions are noted in “VOC emissions analysis” section); three samples were tested under each condition. The rapid test sampling device was based on the Micro-Chamber/Thermal Extractor M-CTE250 (Markes International), connected to a cylindrical volumetric flask to provide moisture and regulate the carrier gas humidity; a handheld thermometer was also used. With constant air flow and low background concentration, the temperature could be adjusted to between 0°C and 250°C and the relative humidity to between 30 and 80 percent. Clean, humidified nitrogen was used as the carrier gas. The total test period was 7 days (168 h). Table 2 shows the operating parameters during the experiment.

Table 2 Micro-chamber/thermal extractor operating parameters of the rapid test method.

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VOC emissions analysis

Thermal desorption sample injector TP-5000 was used to thermally desorb the VOC samples. The temperature was kept at 280°C for 5 minutes, after which the VOC samples were characterized and quantified by DSQ II GC-MS. Table 3 lists the basic GC-MS parameters. This experiment included groups A, B, C, and D; the scheme of the experiment is shown in Table 4.

Table 3 The basic parameters of gas chromatography–mass spectrometry.

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Table 4 Experimental scheme under different methods and environment conditions.

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The effect of environmental factors on the components of VOC emissions

The main components emitted from the three-layer plywood are listed in Table 5. Note that there were more components in the aromatic hydrocarbon and alkane categories than in the others. Components were also found in these categories: alkenes, aldehydes and ketones, esters, and ethers (small amounts).

Table 5 The main components of volatile organic compounds emitted from the three-layer plywood by rapid test method and traditional climate chamber method.

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Figure 1 shows the percentage of different components in stable phase under different environmental conditions and environmental methods. Figures 1a through 1c show the impact that different temperatures, relative humidity levels, and ratios of air exchange rate–to–loading factor had on the percentages of VOC components. The percentages of VOC components changed with changes in single factors. When relative humidity and the ratio of air exchange rate–to–loading factor were held steady, the percentage of arenes and esters steadily increased as the temperature rose. Alkanes reached the highest value at 80°C, but alkenes and aldehydes and ketones reached their highest values at 60°C and 23°C, respectively. Increases in temperature increased the differences among various components (Fig. 1a). When temperature and ratio of air exchange rate–to–loading factor were held steady, the changes in relative humidity had a relatively small effect on the percentage of each component, especially alkanes and alkenes. The changes of VOC components at 50 to 60 percent relative humidity were obviously less than at 40 to 50 percent relative humidity. Aldehydes and ketones reached their highest values at 40 percent relative humidity (Fig. 1b).

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When the temperature and relative humidity were held steady, the percentage of arenes increased as the ratio of air exchange rate–to–loading factor increased, but the percentage of alkenes and of aldehydes and ketones decreased at the same rate as this ratio increased (Fig. 1c). In summary, temperature has the greatest impact on the percentage of different VOC emissions. An increase in temperature and relative humidity can increase the change in the proportion of each component, such as lowering the proportion of aldehydes and ketones and increasing that of esters. Figure 1d shows the percentage of various components detected by two different methods. Although the methods used analogous environmental conditions, the percentages found were obviously different, but the species were the same. Significant differences include the higher percentage of arenes found by the rapid detection method compared with the traditional climate chamber method and the much lower percentage of alkanes detected by the rapid detection method compared with the traditional climate box method. The differences seen in the two detection methods may be explained by the different samples used. The samples for the rapid detection method were cut in round pieces (60 mm in diameter), whereas the samples used in the climate chamber method were cut in rectangular pieces (1,000 by 500 mm). Different sample sizes may affect VOC emissions, with greater stability of VOC emissions seen in larger samples. However, because the rapid detection method is much faster and the difference in results between the two methods is small, the rapid detection method can be used with convenience to monitor VOC emissions in factories.

The effect of environmental factors on VOC emissions

The emissions rate of VOCs gradually decreased with time, until it reached a stable phase. The VOC concentrations sharply decreased from Day 1 to Day 3, after which the rate of decline slowed.

Figure 2a shows the changes in VOC concentrations under different temperatures from Day 1 to Day 7. When the temperature increased, the VOC concentrations also increased. As time went on, the distinctions in VOC concentrations gradually diminished, and they finally reached equilibrium. At 23°C, 60°C, and 80°C, from Day 1 to Day 4, the average rates of decline of VOCs, respectively, were 88.03, 121.53, and 141.82 μg·m⁻³ d⁻¹. Thus, in the first day of the experiment, temperature had a distinct influence on VOC concentration: the higher the temperature, the faster the decline in VOC concentration. Meanwhile, the VOC equilibrium concentration at high temperature was slightly higher than the VOC equilibrium concentration at low temperature. In the later stages, the impact of temperature on VOC concentration became weak; the VOC concentrations in different temperatures gradually reached equilibrium. Temperature has a significant impact on VOC emissions from wood-based panels: as the temperature rises, the thermal motion of VOC molecules strengthens, the VOC diffusion coefficient increases, and the panels' adsorption capacity decreases. The relationship between temperature and VOC diffusion coefficient in the material is simplified as follows:

$$D=D_{\text{ref}}\exp \left|-E\left|T^{-1}-{296}^{-1}\right|\right|$$

where D is the internal diffusion coefficient of the material (m² h⁻¹); D_ref is the internal diffusion coefficient of the material at 23°C; E is determined by the test coefficient, generally 9,000 T; and T is the thermodynamic temperature (K). Temperature influences VOC emissions through its impact on the diffusion coefficient. Based on the mass transfer theory, as a result, the VOC emissions from the panels become fast and massive (Li et al. 2013).

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Figure 2b displays the influence of relative humidity on VOC concentrations. The VOC concentration decreased fastest from Day 1 to Day 2, with decreases of about 200 μg/m³ at each of the relative humidity levels. From Day 2 to Day 5, the relative humidity showed a significant effect on VOC concentration, which declined at a rate that increased as relative humidity increased. Experiments show that, in general, VOC concentrations increase as relative humidity increases, which may be explained by the change in pore structure due to acceleration of hydrolysis. The expansion of pores in the drying layer and hygroscopicity favors the release of VOCs, as shown by Zhu et al. (2013) and Shan et al. (2013).

Figure 2c shows the relation between VOC concentration and the ratio of air exchange rate–to–loading factor. In general, as the ratio of air exchange rate–to–loading factor decreases, the VOC concentration increases. VOC concentration decreased significantly from Day 1 to Day 2, dropping approximately 200 μg/m³ under three different ratios of air exchange rate–to–loading factor. From Day 2 to Day 3, the rate of decrease slowed noticeably, but it still declined slowly. After Day 3, the VOC concentration gradually reached equilibrium. Yu et al. (2006) and Yang et al. (2007) studied the influence of natural ventilation on VOC emissions from interior furnishings and found that increased ventilation would reduce the VOC concentrations, similar to our findings. In short, massive amounts of fresh carrier gas dilute VOC emissions within microcell extraction and enlarge the concentration gradient between microcell extraction and panels, promoting the release of VOCs and reducing the amount of collected release.